1. Field of the Invention
This invention relates to an image forming apparatus such as a copying machine or a printer, and particularly to an apparatus for transferring an image on an image bearing member to a transfer material.
2. Description of Related Art
FIG. 4 of the accompanying drawings shows an image forming apparatus which is the background art of the present invention.
This image forming apparatus has a plurality of photosensitive drums 111a, 111b, 111c and 111d as first image bearing members corresponding to developers (toners) of the first color: yellow, the second color: magenta, the third color: cyan, and the fourth color: black, and an intermediate transferring belt 101 as a second image bearing member, and the intermediate transferring belt 101 is in contact with each of the photosensitive drums 111a–111d in respective primary transferring parts.
The photosensitive drums 111a–111d are disposed along the direction of movement of the intermediate transferring belt 101 in the order of the photosensitive drum 111a for the first color (yellow) located most upstream, the photosensitive drum 111b for the second color (magenta) located nearest to and downstream of the photosensitive drum 111a, the photosensitive drum 111c for the third color (cyan) located nearest to and downstream of the photosensitive drum 111b, and the photosensitive drum 111d for the fourth color (black) located nearest to and downstream of the photosensitive drum 111c. 
Also, the intermediate transferring belt 101 is located at a predetermined process speed in synchronism with the photosensitive drums 111a–111d. 
The photosensitive drums 111a, 111b, 111c and 111d are uniformly charged by contact charging rollers 112a, 112b, 112c and 112d, respectively, and electrostatic latent images are formed on the surfaces thereof by laser beams from scanners 113a, 113b, 113c and 113d modulated by an image information signal sent from a host computer.
These electrostatic latent images reach portions opposed to developing devices 114a, 114b, 114c and 114d by the rotation of the photosensitive drums 111a–111d, and are supplied with toner charged to the same polarity (the minus polarity in this example) as that of the surfaces of the photosensitive drums 111a–111d, and are visualized thereby and become developer images (toner images). The developing devices 114a–114d provided for the respective photosensitive drums 111a–111d are of a two-component developing type, and perform their developing operation by a developing bias comprising an AC voltage superimposed on a DC voltage being applied thereto.
The toner images formed on the respective photosensitive drums 111a–111d are transferred onto the intermediate transfer belt 101 by a primary transferring bias being applied from primary transferring bias sources 116a, 116b, 116c and 116d to primary transferring rollers 115a, 115b, 115c and 115d, respectively, which are in contact with the back of the intermediate transferring belt 101, in respective primary transferring nips formed by and between the intermediate transferring belt 101 and the photosensitive drums 111a–111d. At a stage whereat the intermediate transfer belt 101 has passed the primary transferring nip between it and the photosensitive drum 111d, the formation of a four-color image on the intermediate transferring belt 101 is terminated, and a primary transferring step is completed.
Next, a transfer material M is taken out of feeding means, not shown, and is inserted into a secondary transferring nip portion formed by a separation roller 101c and a secondary transferring roller 102 as a transferring member being brought into pressure contact with each other with the intermediate transferring belt 101 interposed therebetween. At this time, a bias opposite in polarity to the toners is applied to the secondary transferring roller 102 by a secondary transferring bias source 121, whereby the toner image is secondary-transferred from the intermediate transferring belt 101 to the transfer material M.
The transfer material M bearing the unfixed toner image thereon and having left the secondary transferring nip portion arrives at a fixing apparatus 103, and is heated and pressurized thereby, whereby a permanent fixed image is obtained.
Each of the photosensitive drums 111a–111d has an outer diameter of 30.0 mm and has a layer having a photosensitive material applied thereto on an aluminum cylinder.
The intermediate transferring belt 101, as shown in FIG. 2, is passed over three rollers contained in the intermediate transferring belt 101, i.e., a drive roller 101a, a supporting roller 101b and a separation roller 101c. 
As the intermediate transferring belt 101, carbon is dispersed in polyimide and the surface resistivity ρs thereof is adjusted to medium resistance of 1×1012 Ω/□, whereby charges added to the belt with the transferring step or the like can be attenuated without providing any special residual charge eliminating mechanism. Also, the intermediate transferring belt 101 is a single-layer endless belt having a circumferential length of 1000 mm and a thickness of 100 μm.
Each of the drive roller 101a, the supporting roller 101b and the separation roller 101c over which the intermediate transferring belt 101 is passed is a roller having an outer diameter of 29.8 mm and comprised of an aluminum mandrel having a diameter of 24.0 mm and an elastic layer having a layer thickness 2.9 mm. Also, the secondary transferring roller 102 is a roller having an outer diameter 33.0 mm and comprised of an aluminum mandrel having a diameter of 14.0 mm and a rubber layer having a layer thickness of 9.5 mm, and the hardness of this roller is 26° (Asker-C).
The intermediate transferring belt 101 in the present example is of a single-layer construction in which Young's modulus E is 6×109 N/m2. Also, the contact pressure P [N/m2] of the secondary transferring roller 102 in the present example relative to the intermediate transferring belt 101 is 3.3×104 N/m2.
The contact pressure P is given as P=F/(L×W) from the lengthwise width L [m] of the secondary transferring roller 102, the nip width W [m] between the intermediate transferring belt 101 and the secondary transferring roller 102, and the contact force F [N] of the secondary transferring roller 102 relative to the intermediate transferring belt 101.
Also, the nip width W was obtained by applying ink to the intermediate transferring belt 101 and bringing the secondary transferring roller 102 into contact therewith, and measuring the trace of the ink adhering to the secondary transferring roller 102. The nip width W was determined from the average of the widths of the trace of the ink measured at five points in total, i.e., the center of the roller, points of 50 mm from the center toward the right and left lengthwise ends, and a point of 100 mm from the center toward the right and left lengthwise ends. In the present example, L=0.30 m, F=50 N, and W=0.0050 m.
In the above-described image forming apparatus of the electrophotographic type, the stabilization of the conveyance of the transfer material in the secondary transferring step is achieved by securing a wide nip width of 5.0 mm of the secondary transferring roller 102 by the use of a roller of low hardness of 26° (Asker-C) as the secondary transferring roller 102, and prevents the occurrence of a faulty image attributable to the conveyance shock of the transfer material.
On the other hand, a material of high hardness which the Young's modulus E is 6×109 N/m2 is used for the intermediate transferring belt 101 to thereby prevent the destruction of the belt due to the fracture thereof, thereby achieving an intermediate transferring belt having a long life.
Now, when the intermediate transferring belt 101 of high hardness is used as described above, if a roller of low hardness is used as the secondary transferring roller 102, the nip width becomes liable to widen and therefore, this has led to a case where the contact pressure P of the secondary transferring roller 102 assumes a low value and the color unevenness of an image attributable to the secondary transferring step is caused.
This color unevenness caused during the secondary transferring step is considered to be attributable to the unevenness of the surface of the transfer material.
That is, the surface of the transfer material M and the surface of the intermediate transferring belt 101 cannot uniformly contact with each other due to the unevenness of the surface of the transfer material M and the high hardness of the intermediate transferring belt 101 and therefore, depending on locations, air gaps exist between the surface of the transfer material M and the surface of the intermediate transferring belt 101.
At locations whereat air gaps exist between the toner layer on the intermediate transferring belt 101 and the surface of the transfer material, a transferring electric field originally applied to only the toner layer is divided by an air layer, whereby the electric field applied to the toner layer is weakened, and the amount of toners residual on the intermediate transferring belt 101 becomes great. This is because unless the reversal of the polarity of the toners occurs, the amount of toners transferred from the intermediate transferring belt 101 to the transfer material M becomes greater when the transferring electric field applied to the toner layer is greater.
It is considered that if as described above, the contact state between the surface of the transfer material M and the surface of the intermediate transferring belt 101 is partly non-uniform, the untransferred toners also become non-uniform. The difference by the portions of the toners not secondary-transferred at this secondary transferring step is considered to be the cause of the occurrence of the uneven colors of an image.
Consequently, in the above-described example, a roller of low hardness is adopted as the secondary transferring roller 102, whereby the contact pressure P assumes a low value and it becomes difficult for the unevenness of the surface of the transfer material M to follow the surface of the intermediate transferring belt 101, whereby the contact state between the surface of the transfer material M and the intermediate transferring belt 101 becomes non-uniform and the residual toners on the intermediate transferring belt 101 also become non-uniform and therefore, the uneven colors are considered to occur.